Why are you targeting children? the people who will be more likely to buy horse treats, are the adults who usually own those horses, or the parents. If you look at your local tack store, or even online-there are some books available with recipes to make horse treats. Im sure if you do some searching online, you'll also find all sorts of horse treat recipes.
I would have fun with it. Make them different colors, different flavors. Most ingredients need to make them you either get through your local horse feed store and at your local grocery store. If you make lots and make them affordable, I think people would rather buy fresh, home made ones as opposed to store bought ones. I certainly would rather buy ones a local horse owner makes than the dry bags you get at most tack stores.